Plasma levels of high density lipoprotein (HDL) relate inversely to atherosclero sis risk. Variability in the fractional catabolic rate (FCR) of apo A-1, the major HDL apolipoprotein, is a strong inverse correlate of HDL-cholesterol (HDL-C) levels, suggesting that apo A-1 FCR may regulate HDL-C. The size of apo A-1 containing particles can predict apo A-1 FCR, and studies now suggest that an important factor in this relationship may be the presence of apo A-1 on plasma lipoprotein particles smaller than typical HDL.